Method of making arsenates



Patented Dec. 1, 1925.

UNITED STATES "1,564,093 PATENT OFFICE;

Emoms AUSTIN MITCHELL, OF BIbGEWOOD, NEW JERSEY, AND KEBE "130L181, 01NEW YORK, N. Y.

METHOD OF MAKING ABSENATES.

Application filed July 91, 1922. Serial F0. 578,627.

Toallwhomz'tmaycomem:

Be it known that we, THOMAS Ausrm Mrrormnn and Kenn TOABE, citizens ofthe United States of America, residing at Ridge- 5 wood, in the countyof Bergen, State of New Jersey, and New York city, in the county of NewYork and State of ,New York, respectively, have invented certain new anduseful Improvements in a Method of Making Arsenates, of which thefollowin is a full, clear, and exact specification.

ur invention relates to the manufacture of arsenates and moreparticularly to an improvement over the methods set forth in the priorpatents to T. A. Mitchell, Numbers 1,183,315 and 1,183,316, of May 16,1916. a

In the first Patent #1,183,315, Mitchell has disclosed a method ofmaking arsenate by treating arsenic trioxid with stron nitricacid in thepresence of metallic lea According to this method, the trioxid, known aswhite arsenic, is oxidized by nitric acid to arsenic acid. At the sametime, lead is 25 dissolved as lead nitrate which in solution reacts withthe arsenic acid to form a precipitate of lead arsenate. Since thisreac-' tion depends upon the continuous addition of nitric acid to amixture of lead and the trioxid, the amount of water in the bathincreases and the concentration of .thearsenic acid solution thereforeprogressively decreases. Coincident with this increasing dilution, wefind that while at the beginning of the process a finely divided andflocculent preclpitate is obtained, the physical condition of thearsenate changes as the reaction proceeds and the product soon appearsas a heavy fla scale adherin tot e metallic lead, which orm is not weladapted for commercial use.

In accordance with the method set forthin the other patent, the leadarsenate is formed by em loying originall the higher oxidation pro uct,arsenic aci and treating an a ueous solution thereof with nitric acid int e presence .of metallic lead. The concentration of the solution ofarsenic acid is at first high, but as the reaction proceeds and arsenicis removed from the solution, the concentration is likewise diminishedwith the objectionable results as noted in connection with thefirstpatent, i; e., oh-

- taining an undesirable form of precipltate.

hereto.

In the second method, one may resort to heat to evaporate the waterandmaintain the solution concentrated but this obviously has manydisadvantages, due not onl to the expensive procedure lnvolved and. t ewear and tear on the apparatus but also to the fact that the unit cannotwork to full capac- 1ty or efliciently, as is particularly true towardthe end of the rocess when the amount of liquid is-sma and the mass ofthe solids proportionatel large.

It is accordmgly an o ject of our inventlon to remedy such diflicultiesand avoid the formation of objectionable types of preci itate, andparticularly to provide a simp le, economical and practicable method ofform:- mg metallic arsenates as a finely divided or flocculentprecipitate which come down 0011- tmuously 1n a substantially uniformphysical condition.

, With this and other objects in view as will apparent in the followingdisclosure, our invent on resides in the steps of the process set forthin the following description and covered by the claims appended In the.course of our experimentation we have discovered that it is desirableand feasible to start initially with a concentrated arsenic acidsolution and to maintain its concentration during the formation of thearsenateby replenishing the arsenic in solution as the arsenaterecipitate is formed,

and we may acc'omplis this by continuously 1 adding arsenic trioxid tothe bath and there oxidiz it to the higher form for reaction with aesired metal element to'form the arsenate. The rates of oxidizingarsenic trioxid and of converting it to the arsenate are soregulated asto keep the concentration of the arsenic acid solution substantiallyuniform. To this end we introduce the desired metal element into thereaction zone at such a rate as to remove arsenic acid from the solutiononly as fast as it is being re leni'shed by oxldation of the-arsenictrloxi Themetal element maybe, introduced into the bath in the form of.a metal,

such as lead, in a finely divided state, or it may be in a compound,such as lead oxid. For the purpose of bringing-such metal element intothe reaction, the bath contains a suitable solvent for the materialemployed. and we prefer that the solvent be capable 'ter alone.

As one application of our invention we may form a metalllc arsenate,such as lead arsenate, by feeding a mixture of arsenlous oxid and leadin a suitable form, such as metallic lead or lead oxid, but preferablymetallic lead in a finely divided condition, to an aqueous bath ofconcentrated arsenic acid containing the necessary quantity of nitricacid. The arsenic acid solution (ordinarily to 68 Baum) is utilized atsuch a concentration as is sufficient to prevent the precipitate comingdown in other. than a finely divided form but it is preferably weakerthan a saturated solution, since the latter tends to crystallize out atthe working temperature .of the process and necessitate additionalwashing of the final product to free it from the unconverted material.The concentration of the bath may be maintained uniform by regulatingthe rates at which the various reagents are fed thereto. Introduction ofan excess of lead, depending upon its rate of solution, tends towithdraw arsenic acid from the bath and reduce the concentration, whilethe addition and oxidation of more trioxid replenishes the arsenic.Thus, we may, by utilizing suitable apparatus, as is well-known in theart, effect a continuous operation of the process by starting with asolution of the desired acid strength and concentration of arsenic acidand then feeding the dry ingredients thereto at the required rate, whilecontinuously removinlg the precipitate and nitrogen gases for fi trationand reuse respectively. v

Asa specific example of a methodfor making lead arsenate the procedureillustrated diagrammatically in the drawing may be adopted. We dpreparean aqueous solution of concentrate arsenic acid, referably 68 B., andadd to 500 poun s of this solution 100 pounds of nitric acid of 42 B.This mixture is'run into the reaction tank where finely divided lead andarsenious oxid are added. These two ingredients are preferably mixedtogether in the dry state in the proportions of 136 pounds of finelydivided lead and 65 pounds of arsenious oxid. This mixture is in theproper proportions for the reaction and it is added at such" arate as toprevent substantial change in the concentration of the arsenic acidsolution which would affect the form of the precipitate. The rate offeeding and the temperature are also regulated to prevent foaming andthe formation of undesired products. The temperature at which thereaction should be conducted depends of course upon the nature of thereagents employed and the product desired. We find that in making leadarsenate accordin to the above method, it is desirable to limit thetemperature to between 80 C. and 120 C.

Any suitable method of separatin the precipitate from the liquor and forrec aim;

mg the various reaction products may beemployed, as is well known inthis art. As illustrated in the drawings, we may effect tlns separationby counter current lixiviation, such as may be had by employin the .Dorrclassifying systems In accor ance with this process, the lead arsenateand the reaction liquor are treated in the classification apparatus witha counter current of wash water, whereby the arsenic acid is washed fromthe precipitate. The acid may be conc entrated, as by evaporation, forreturn to thereaction tank. If desired, the nitrogen oxid gases may becollected and nitric acid regenerated therefrom for return to theMoreover this'material has substantially the same uniform qualities atall stages in the reaction. While we prefer to so combine theingredients and carry on the process as to form a compound of theformula of PbI'IAsO it is to be noted that the pro-. portions of arsenicacid and lead may be varied to produce either the ,tri-, di-, ormonobasic arsenate or mixtures of these salts as may be desired.

It is furthermore clear that this method may be varied b suitablechanges such as are within the nowledge of one skilled in the artwhereby we may produce various other'arsenates, such as those of copper,zinc, iron and tin; and that the metal ions may be introduced bydissolving in the bath either a metal or any suitable compound thereof.

Having thus described our invention what we claim as new and desire tosecure vby nate comprising the steps of oxidizing arsenic trioxid toarsenic acid in the presence of a concentrated solution of arsenic acidand simultaneously adding a metal element of the desired arsenate andprecipitating the metal arsenate b reaction with the arsenic acid, andregulating the rates of forming the arsenic acid and of precipitatingthe metal arsenate so as to maintain a substantially uniform and highconcentration of the arsenic acid.

2. The method of forming a metal arsenate, comprising the steps ofpreparing a bath containing a highly concentrated solution of arsenicacid and a reagent capable of oxidizing arsenic trioxid, and addingarsenic trioxid and the metal element of the desired arsenate at suchrates as to substantially maintain the concentration of the arsenic acidwhile forming a metal arsenate therefrom.

3; The method of forming a metal arsenate, comprising the stepsvofadding arsenic trioxid and a metal corresponding to the desired arsenateto a bath containing arsenic acid and a reagent capable of oxidizing thearsenic trioxid and dissolving the metal, and regulating the addition ofthe metal and the arsenic trioxid and the rates of reaction to maintainsuch a concentration of the arsenic acid solution asito obtain a finelydivided precipitate of substantially uniform quality. I v

4. The method of forming lead arsenate, comprising the steps ofpreparing a bath of a concentrated solution of arsenic acid and nitricacid and adding finely divided lead and arsenic trioxid at such rates asto maintain the concentration of arsenic acid sub-- bath containing aconcentrated solution of arsenic acid, a reagent capable of oxidizingarsenictrioxid and a solvent for the metal of the desired arsenate,adding the metal and arsenic trioxid at such rates as to maintain such aconcentration of the arsenic acid while forming the metal arsenate aswill give a precipitate of uniform quality, replenishing the oxidizingreagent, and continuously removing the precipitated arsenate from thesolution.

7. The method of making a lead arsenate,

comprising the steps of preparing a bath contalning a concentratedsolution of arsenic' acid and nitric acid, adding finely divided leadand arsenic trioxid at such rates as to maintain the concentration ofthe arsenic acidwhile forming lead arsenate, replenishing the nitricacid and continuously removing the precipitated arsenate from thesolution.

Signed at New York city, New York, this 19th day of J uly, 1922.

THOMAS A. MITCHELL. KEBE TOABE.

